Wireless radio frequency (RF) receivers are used in a wide variety of applications such as smart metering, remote control, home security and alarm, telemetry, garage and gate openers, remote keyless entry, and the like. As used herein, a “radio frequency” signal means an electrical signal conveying useful information and having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. One common type of RF receiver is a frequency-shift keying (FSK) receiver that is compatible with the industrial, scientific and medical (ISM) radio bands in the 119 to 1050 megahertz (MHz) range. ISM radio bands are portions of the radio spectrum reserved internationally for the use of RF energy for industrial, scientific, and medical purposes other than communication.
Certain wireless communication standards define a preamble for a wireless packet, which is a predefined data pattern that a receiver can use to detect and settle its control loops. These control loops include the Automatic Gain Control (AGC), Automatic Frequency Compensation (AFC), and Bit Clock Recovery (BCR). After the receiver detects the end of the preamble, the receiver is prepared to receive a full packet including payload data. While many standards define a relatively long preamble pattern length (for example about 32 preamble bits in an alternating . . . 1010 . . . pattern) to provide enough time for these loops to settle, the N-mode of the wireless version of the Meter-Bus (M-Bus) standard, no. EN 13757-4, defines a relatively short preamble (about 16 bits in an alternating . . . 1010 . . . pattern). Although the shorter preamble provides an opportunity for the receiver to work faster and to take advantage of corresponding power savings, the shorter preamble also increases the burden on the receiver to reliably detect the signal arrival of the preamble and to settle its control loops in time.
Known receivers use a preamble detector to detect the arrival of a frame. In response to the preamble detector signaling the detection of the preamble, the receiver begins looking for the next portion of the frame. In the M-bus frame protocol, this next portion is a synchronization (SYNC) word. However under certain circumstances, the preamble detector can occasionally provide a false trigger, such as when a co-channel continuous wave (CW) tone is received at around sensitivity of the preamble detector, or when certain noise patterns are received. Known receivers solve this problem by using a timer to limit the amount of time the receiver searches for the SYNC word. If the timer expires before the SYNC word is detected, the receiver concludes that the preamble was erroneously detected and re-initializes itself to again start searching for a preamble. However a problem occurs when a real frame arrives after a false positive preamble detection but before the receiver has had a chance to re-initialize. In this situation, a real frame can be lost.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.